Project Summary/Abstract Vascular changes are increasingly common with age and a risk factor for some forms of cognitive impairment including Alzheimer's disease (AD). In particular, alterations to cerebral perfusion have been widely reported, with decreased blood flow observed in cortical and limbic regions associated with the accumulation of amyloid beta plaques and tau-containing neurofibrillary tangles. Though not expressed by cells that make up cerebral vasculature, the neuronal protein tau has been observed inside of the vascular endothelium in post mortem tissue sections highlighting the intriguing possibility that it may directly affect endothelial cells. In mice, overexpression of tau appears to alter vascular density, reduce average vessel diameter particularly within the capillary bed, and increase the incidence of vessels blocked by leukocytes. These findings could explain, in part, altered cerebral perfusion changes in Alzheimer's disease patients, which may be a key contributor to cognitive decline. The overall goal of this project is to determine how these tau-induced vascular alterations observed in mice affect cerebral perfusion and, ultimately, neurodegeneration. Experiments will be carried out in aging tau overexpressing mice carrying the human P301L mutation (Tg4510 line) and advanced in vivo imaging methods. To assess cerebral perfusion, key measurements will be made in awake mice by two-photon microscopy including red blood cell flow, cerebral oxygenation, and of the hemodynamic response using a visual stimulation paradigm. If tau induces early vascular dysfunction, it will be evident by a reduction in hemodynamic response and poor tissue oxygenation, which will lead to subsequent neuron loss. Further, reduced perfusion could be partially explained by the observation of vessels block by leukocytes. In a second set of experiments, a novel cranial window port method will be used to administer labeled tau and protein directly into the parenchyma to determine if tau is sufficient to increase expression of cell adhesion molecules (CAMs) and induce blood vessel blockages by leukocytes. These investigations will also make use a doxycycline repressible promoter to turn off tau expression in mice and determine if changes in endothelial CAM expression and leukocyte blockage is reversible, which is an important consideration for the development of targeted therapeutics. Findings from these studies will further our understanding of tau biology in non- neuronal subtypes as well as the impact of vascular alterations on brain health more generally, which has broad implications beyond Alzheimer's disease. Finally, by utilizing magnetic resonance imaging methods developed for assessing tumor microvasculature in vivo, we have a unique opportunity to validate the use of these techniques in a neurodegenerative disease model such that we can directly translate the transgenic mouse work to human AD research.! !